The sum of the interior angles of a polygon of n sides is (n-2)*180 degrees. Here n=5, so that sum is (5-2)*180 degrees, or 3*180 degrees, or 540 degrees.
Adding up the 5 interior angles shown, we get 100+102+108+121+x = 540 (degrees). This reduces to 431 + x = 540 (degrees). Thus, x = 540-431 degrees, or x = 109 degrees (answer).
